This invention relates to sheet material having a heat-sealable stratum, and is especially concerned with sheet material useful in the manufacture of tamper-resistant seals for capped bottles or jars.
For several years a number of products packaged in bottles or jars having screw-on lids or caps have been provided with an additional membrane-like seal that protects the contents when the cap or lid is removed. This membrane, which is approximately the same diameter as the outer diameter of the container mouth, is adhered to the lip of the container and cannot be removed without evidence of tampering. Additionally, this inner seal excludes air from the contents of the opened container and helps prevent leakage of packaged liquids.
Because of its simplicity of application, one highly commercial form of inner seal includes a layer of pulpboard bonded with microcrystalline wax to an aluminum foil, the exposed surface of the foil being provided with a heat-sealable coating which is tack-free at room temperature. In use, the pulpboard backing is permanently glued to the inner upper surface of a threaded cap or lid, which is then mounted on the complementarily threaded mouth of a filled container. The cap is then passed through an induction heating unit, which rapidly heats the aluminum foil, simultaneously melting the microcrystalline wax and the heat-sealable coating. The melted wax is quickly absorbed by the pulpboard backing, greatly reducing the strength of bond between the backing and the aluminum foil, so that when the cap or lid is removed, the pulpboard backing remains inside the cap or lid, while the aluminum foil remains firmly sealed to the mouth of the container. Details of this process are set forth in considerable detail in U.S. Pat. No. 2,937,481.
Although the structure just described has been used with great effectiveness on containers formed from polyvinyl chloride, polyester, polyethylene, polypropylene and acrylonitrile:methylacrylate copolymers, it has been much more difficult to obtain satisfactory bonds to glass containers. Prior to the present invention, the heat-sealable adhesives for bonding to glass have included ethylene:vinyl acetate copolymers, polyester resins, and polyionomer resins, available from E. I. duPont de Nemours & Co. under the trade designation "Surlyn". While all of these adhesives are capable of being heat sealed to glass containers, the bond is weak enough that the inner seal can be peeled away without difficulty. As a result, it is believed that no one has heretofore been able to achieve a firmly bonded metal foil inner seal on glass containers.